> Readers don't block writers, writers don't block readers in PostgreSQL.
>
> pg_dump is a reader.
>
> --
> Holger Jakobs, Bergisch Gladbach, Tel. +49-178-9759012

Additionally, I've done some stress testing and found that pg_dump puts surprisingly low load on our dbs. Of course, like everything else, this dependent on your specifics--after all the dump will require reading all rows, so for instance if you're disk-bound, you could see a performance hit. But generally, if your db is running in a reasonably "healthy" performance range and not already close to limits, pg_dump won't have a performance impact visible to users.